Underground formations encountered during exploration and production of a well may exist at elevated pressures. In many instances, the pressures are substantial enough to produce an elevated pressure at a wellhead. Failure to control such pressure differentials could result in an undesirable situation referred to as a blowout—an uncontrolled flow of reservoir fluids into the wellbore, and sometimes catastrophically to the surface.
Typically, a well might be fitted with a wellhead fixture to isolate wellbore pressures from an ambient pressure at an open end of the wellbore. During exploration and production, however, there remains a need to at least periodically install and/or remove downhole devices from the well. For example, logging tools designed to evaluate a formation and/or well conditions must be inserted into the wellbore, lowered to various depths as may be required during exploration, and later removed from the wellbore, without jeopardizing crew, equipment, or production of the well. Presently, transfer of such logging tools through an open of a well under pressure can be accomplished using specialized fixtures and techniques capable of maintaining a pressure barrier at the wellhead. One such class of fixtures is known generally as a Christmas tree, including a configuration of valves and access fittings. Another such class of wellhead fixtures is known generally as blowout preventors (BOPs). Either class of wellhead fixtures can be configured with facilities to enable safe access for well intervention apertures. For example, BOPs can include an open channel with one or more reversibly sealable elements configured to open to allow passage of the logging tool and closing thereafter to form a pressure barrier.
The process of putting drill pipe or other downhole devices into a life well under pressure when BOPs are closed and pressure is contained within the well is referred to as snubbing. If the well has been closed with a so-called ram-type BOP, larger diameter features of the downhole devices, such as tools or joints will not pass by the closed ram element. To keep the well closed another ram-type BOP or an annular BOP is included in series. The first ram element must be opened manually, then the downhole device lowered until the larger diameter feature is just below the ram element, and then closing the first ram element again. The second ram element is then opened allowing the larger diameter element to pass. This procedure is repeated whenever a larger diameter feature, such as a tool or tool joint must pass by a ram-type BOP. Exercising such care in dealing with larger diameter features by snubbing is generally a time consuming proposition.
If only an annular BOP has been closed rather than the ram-type BOP, the drill pipe or other downhole device may be slowly and carefully lowered into the wellbore, since the annular BOP opens slightly to permit the larger diameter feature to pass through. In snubbing operations, the pressure in the wellbore acting on the cross-sectional area of the tubular element (i.e., downhole device) can exert sufficient force to overcome the weight of a drill string, so the string must be pushed (or “snubbed”) back into the wellbore. Such thrust can be provided by a coil tubing unit pushing to a proximal end of a tool or axial array of tools within the wellbore. Such an axial array of tools is referred to as a tool string.
Applying downhole axial thrust to such an elongated tool or string of tools generally requires the use of a rig or derrick providing lateral support to the tool or string of tools suspended above the wellhead fixture. Such strings are typically assembled vertically above a wellhead fixture before insertion, requiring tall rigs. The rig itself is constructed above the open end of the wellhead fixture and directed along the wellbore axis and may extend from 10 to 100 feet or more, depending upon the length of the tool or tool string. An array of multiple interconnected tools is referred to as a tool string. Such strings are typically assembled vertically above a wellhead fixture before insertion, requiring tall rigs. Unfortunately, construction of such a rig or derrick adds to time and complexity on-site during any such deployment and extraction procedure. The rigs must be provided, constructed, used, deconstructed and removed. Such on-site access time can be quite expensive, particularly for offshore applications, thus any procedures leading to delay, such as snubbing and rig construction, are highly undesirable.